One existing type of switch is a radio frequency (RF) micro-electro-mechanical system (MEMS) switch. This existing type of switch typically has a substrate with two conductive posts spaced apart on the substrate. A conductive part (e.g., electrode) is provided on the substrate between the posts, and is covered by a layer of a dielectric material. A flexible and electrically conductive membrane extends between the posts, so that a central portion of the membrane is located above the conductive part on the substrate. An RF signal is applied to one of the conductive part and the membrane.
In the deactuated or non-actuated state of the switch, the membrane is spaced above both the conductive part and the dielectric layer covering it. In order to actuate the switch, a direct current (DC) bias voltage is applied between the membrane and the conductive part. This bias voltage produces charges on the membrane and the conductive part, and the charges cause the membrane and conductive part to be electrostatically attracted to each other. This attraction causes the membrane to flex, so that a central portion thereof moves downwardly until it contacts the top of the dielectric layer on the conductive part. This is the actuated position of the membrane.
In this actuated state of the switch, the spacing between the membrane and the conductive part is less than in the deactuated state. Therefore, in the actuated state, the capacitive coupling between the membrane and the conductive part is significantly larger than in the deactuated state. Consequently, in the actuated state, the RF signal traveling through one of the membrane and conductive part is capacitively coupled substantially in its entirety to signals traveling along the other part.
In order to deactuate the switch, the DC bias voltage is turned off. The inherent resilience of the membrane then returns the membrane to its original position, which represents the deactuated state of the switch. Because the capacitive coupling between the membrane and conductive part is much lower in the deactuated state, the RF signal traveling through one of the membrane and capacitive part experiences little or no capacitive coupling to signals traveling along the other part.
In certain applications, the ratio of capacitance in the actuated state to capacitance in the non-actuated or default state can be very important. In general, the greater the capacitance ratio is, the greater the bandwidth is that can be provided by the switch. The non-actuated capacitance, or off-capacitance, is a function of the switch membrane and parasitics when the membrane is in the non-actuated position. The actuated capacitance, or on-capacitance, is a function of the metal-insulator-metal (MIM) capacitor formed when the membrane snaps down to the actuated position on top of the dielectric covering the electrode. To provide a RF MEMS switch with better performance characteristics, it is therefore desirable to increase the on-capacitance of the switch.